It’s one of the basic lessons of economics, and the same principle can be applied to tolls, as the Pennsylvania Turnpike is finding out.

Motorists using the turnpike will face their fifth toll increase in as many years when rates climbed by 10 percent for cash-paying customers (2 percent for those using EZ-Pass) on Sunday.

A recent investigation by the Pittsburgh Tribune-Review examined how those increases have affected truck traffic on the highway — suggesting that the higher tolls have caused trucks to divert onto local roads in western Pennsylvania instead of using the turnpike: